Sheet materials may be processed by several means such as stamping, joining cut pieces, thermoforming and folding. The folding process requires very little in-plane deformation of the sheet, and offers manufacturing advantages in many applications. The resulting surfaces may be modeled as zero curvature surfaces (Gauss curvature), with minor error due primarily to the radius of curvature along the fold creases.
Recently in U.S. patent application Ser. No. 09/952,057 filed Sep. 14, 2001 by Kling (hereinafter “Kling”), which is herein incorporated by reference, a vast array of doubly periodic folded patterns (DPFs) was invented that demonstrate diverse application for the DPF including for laminated core materials in rigid panels. Also processes for continuously producing DPFs have been disclosed in Kling. As sheet material may often be delivered in very long sheet on a roll, the advantages of the continuous manufacturing process for sheet materials include speed and economy. Several preferred machine designs will be discussed herein.
There are two general methods for continuous no-stretch processes for producing zero-curvature structures, namely, a gradual folding technique and a bunch and crunch technique.
To design the gradual folding process one may take a long folded sheet of the desired geometry, and unfold one end by pulling apart the pattern while applying force to flatten it. This may be done either by actual experiment or by calculation or by simulations. FIG. 1 shows a numeric estimate of such a partially folded DPF. The folding pattern is then sampled at incremental positions, starting with the flattened end and proceeding to the fully folded end. Rollers pairs with the pattern negatively imprinted on them in each of these positions are arranged in analogous sequence as shown in FIG. 2. Alternatively stamping dies in the sampled patterns may be positioned in sequence. The material is fed through. Potential problems with this method include the difficulty in changing tooling for new product specifications and the length of the roller sequences needed to draw the material in laterally.
The bunch and crunch method is designed by taking a folded sheet in the desired specification, measuring the lateral contraction ratio, designing a pre-gathering (bunching) method for giving the sheet longitudinal corrugations with the same lateral contraction ratio as the desired folded pattern, and designing pattered rollers with the folded sheet negatively engraved on them, and linking these so the corrugated material with the same contraction ratio of the folded sheet is fed through the patterned rollers. Note the final roller in the bunch and crunch method has the same geometry as the final roller in the gradual folding method, namely the roller is a circumferential expression of the desired pattern.
One problem with the bunch and crunch method is related to the longitudinal (machine direction) movement of the sheet as it is folded in the rollers. The material does all of its longitudinal contraction in the transition zone that extends from just before it is fed into the rollers to approximately the midpoint between the rollers on the plane containing their two axis.
The contact between the extreme edge of the teeth of the roller and the local position on the sheet will be discussed. In the plane containing the two roller axis the teeth extreme edges move nearly tangentially to the crease position. A schematic is shown in FIG. 3 and FIG. 4. In the FIG. 4, which represents the sheet in an enlarged view of FIG. 3, the dotted line represents the midplane containing the rollers' axis. One should note that the spacing between the crease points in the sheet changes as the sheet advances toward the midplane. The segments in the zig-zag line are different lengths until it meets the midplane. This means the crease locations in the sheet relative to the sheet roll or migrate through the sheet. Without in-plane distortion in many cases it is actually impossible for the sheet to slide over the successive teeth to reach the desired folded pattern because of the friction in the rollers and the difficulty in having creases migrate through the sheet material. As explained in Kling, the sheet was pre-gathered, to eliminate the problem of lateral teeth slippage over the roller teeth, but the longitudinal slippage problem still remains. In metals for instance, forcing excessive migration of creases under tension generally damages the sheet, especially at the fold vertices, and often the depth of the pattern must be severely limited to avoid punctures or tears.
With these two methods as backdrop, we describe first an improvement to the roller design in U.S. patent application Ser. No. 10/755,334 filed Jan. 13, 2004 by Kling, Basily and Elsayed (hereinafter “Kling et al.”), which is incorporated herein by reference, to resolve the longitudinal slippage problem, and then several new machines designs and processes are described.